[web-devel] web-devel Digest, Vol 71, Issue 1

Julian Arni jkarni at gmail.com
Fri Oct 23 23:27:00 UTC 2015 

So let's say we have:

/auth ---> application mounted here
POST /auth/subauth  --> normal handler
POST /auth                 ---> normal handler

If a GET to "auth/subauth" arrives, it gets dispatched to the Application.
If the Application doesn't have a subauth endpoint, it 404s. But the
response we want is 405, since there is something that matches the URL (but
not the method). If the Application responded with essentially any other
error besides 404, however, or with a successful response, we don't want to
override it with a 405.

Moreover, if a POST to /auth arrives, where do we send it to - the normal
handler or the Application? The handler just because we know that matches?
Or the Application because it showed up first in the code, even though it
might not match? Essentially, we just don't know how to route, given that
everything inside Application is opaque.

We can try sending things to the Application and checking the first line of
the response. However, not all 404s are created equal. If the handler
decides to 404 because the request body contains an ID for a User in the
database that happens to not exist, rerouting would probably be misleading
(returning a 405 would definitely be). Also, that solution just generally
feels very hacky!

To some extent we can just warn in the documentation against handlers that
might overlap with the mounted Application (or even do static checks to
prevent that), but this isn't very nice when e.g. one web framework is
meant to "stand in front" of another, in the sense that anything that
doesn't get matched by the one framework should fall through into the other
(the mounted Application one), which I imagine being a decently common
use-case. (Because the problem also exists in that direction, where the
Application is the last tried thing - if we reroute to the Application
because the best we got was a 405, the Application might respond with a
404, confusing everybody...) It also doesn't really allow for one framework
handling a GET, and another (via application) a POST on the same URL, say,
without the same issues. Anyhow, these things seems like something to
consider if integrating between frameworks via Application becomes a more
heavily used feature.

Not sure if this makes it any clearer - let me know if not!


On Sat, Oct 24, 2015 at 12:54 AM, Greg Weber <greg at gregweber.info> wrote:

>
>
> On Fri, Oct 23, 2015 at 8:00 AM, Julian Arni <jkarni at gmail.com> wrote:
>
>> Thanks again for starting the discussion!
>>
>> As I mentioned, for Servant I think it's unlikely we'd completely move
>> away from our current handlers, since we don't want to lose all the type
>> information we have, and don't want to impose such information on others.
>> So I'm hoping the next best thing might be achievable - allowing the
>> new-style handlers to be run directly, and having conversion functions from
>> our current handlers to the new style. I think the first is easy, and the
>> second possible. Since there isn't enough information at the value-level to
>> establish where it is our handlers are getting their arguments from (query
>> string, headers, request body, etc.), we'd need a Proxy argument for the
>> relevant part of the api. Something like:
>>
>> convert :: (CanConvert api x) => Proxy api -> x -> NewHanderM ()
>>
>> Where 'convert' would need to walk down the arguments of x and pull
>> arguments from the appropriate places. E.g., if
>>
>> x    ~ Int                      -> Person
>> -> ExceptT ServantErr IO (),                  and
>> api ~ Header "H" Int :> ReqBody '[JSON] Person -> Post '[] ()
>>
>> we'd have 'convert' conceptually doing: convertExceptT $ liftM2 x
>> (parseHeader <$> getHeader "H") (decode <$> getBody), where getHeader and
>> getBody are functions in the new handler monad (/class).  (This isn't quite
>> right, since decoding failures need to be handled).
>>
>>
>> Though as I mentioned before, since for servant at least in one direction
>> it'd be a conversion rather than direct usage of the new handler, and since
>> we can already convert any servant handler plus appropriate Proxy into an
>> Application, and can use any Application as a handler, for us it'd be a lot
>> easier to just use Application. As far as I can tell, this is also true in
>> a couple of other frameworks. Indeed, one of the great things about the
>> Application signature, in my opinion, is that it works well both as the
>> signature for an entire Application and for a handler (that gets a modified
>> request). The only problem we've come across with that idea is that the
>> type of failure of the handler is opaque to the outer framework - so e.g.,
>> there's no easy way to tell whether re-routing needs to be triggered
>> because the handler failed to match. This, however, would still be a
>> problem in the model mentioned above. Having something like Application,
>> but with return type IO (Either ReroutePlz ResponseReceived), would likely
>> suffice (as would settling on an exception type).
>>
>
> I think that for an Application to be shared it needs to be mountable to
> an arbitrary route (e.g. "/auth"). And any request that matches the mounted
> route should be dispatched to just the mounted Application. Inside that
> mounted route the Application can then do additional routing. But if it
> does not match, it should return a 404, and that should be sent all the way
> to the client. Or is this too restrictive? What exactly is the need to
> interpret a 404 as meaning "try a different route"?
>
>
>>
>> On Thu, Oct 22, 2015 at 2:00 PM, <web-devel-request at haskell.org> wrote:
>>
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>>>
>>> Today's Topics:
>>>
>>>    1. More collaboration in web development (Michael Snoyman)
>>>
>>>
>>> ----------------------------------------------------------------------
>>>
>>> Message: 1
>>> Date: Thu, 22 Oct 2015 08:13:31 +0300
>>> From: Michael Snoyman <michael at snoyman.com>
>>> To: web-devel <web-devel at haskell.org>, haskell-wai at googlegroups.com,
>>>         "yesodweb at googlegroups.com" <yesodweb at googlegroups.com>
>>> Subject: [web-devel] More collaboration in web development
>>> Message-ID:
>>>         <CAKA2JgLpqbOiHVY+vH4vdtUjvY7O-Mb7e2x+ms=B1b=
>>> NFqA-+g at mail.gmail.com>
>>> Content-Type: text/plain; charset="utf-8"
>>>
>>> I originally kicked this off as an email to the maintainers of some of
>>> the
>>> other WAI web frameworks. After some initial brainstorming and some broad
>>> consensus to find some more shared common ground, we decided to open this
>>> up to a broader discussion.
>>>
>>> I think the primary forum for this discussion will likely be the
>>> web-devel
>>> mailing list, though I'm cross-posting to haskell-wai and yesodweb as
>>> well
>>> for those interested. Please feel free to share with other mailing lists
>>> also. (We debated whether to use web-devel or haskell-wai for this
>>> discussion and didn't come to a decision, I'm fine moving it elsewhere.)
>>>
>>> Based on the discussion, it seems like our best next step will be putting
>>> together a package containing some form of a Handler monad that could be
>>> shared by multiple web frameworks, hopefully making it easier to write
>>> code
>>> that can move between various frameworks. By contrast, we also seem to be
>>> in agreement that the high level routing code will end up being
>>> framework-specific, though we may be able to share some common low-level
>>> Trie-based dispatch code.
>>>
>>> I'll leave it to other participants to restate points they made in the
>>> initial private discussion.
>>>
>>> * * *
>>>
>>> Hi all,
>>>
>>> When I was in San Francisco a few months ago, Greg and I discussed the
>>> state of Haskell web development. It's clear that there's a lot of great
>>> stuff going on. Both of us have a strong desire to find as much common
>>> ground for all web framework authors to work on as possible, without
>>> detracting from meaningful distinctions between the various frameworks.
>>>
>>> Greg and I have discussed this point off and on for the past few months,
>>> and haven't come up with very many concrete ideas. The recent work on
>>> http-api-data[1] would be one possible example of collaboration. Other
>>> possible ideas that Greg and I brainstormed were some kind of shared
>>> library for efficient dispatch (that could be used under the surface for
>>> the various high-level interface different libraries already provide), or
>>> some kind of a common Handler-like monad (which from what I've seen many
>>> people seem to reimplement all over the place). Some concrete benefits we
>>> can see from this:
>>>
>>> * Previously, Persistent users were complaining that their types did not
>>> serialize in servant. http-api-data addresses this.
>>> * A common Handler would allow a higher layer abstraction for web apps,
>>> much like WAI provides at a low level. With something like that, sharing
>>> code like authentication logic becomes a reality
>>> * We already have precedence for other such shared libraries:
>>> client-session, server-session, cookie, and authenticate, as a few
>>> examples.
>>>
>>> These are just ideas to kick off some thoughts, I'm sure some discussion
>>> could bring out some better ideas. Overall, it seems like we have a lot
>>> of
>>> opportunity to improve the situation for everyone if we focus on such
>>> collaboration. Having a shared low-level HTTP interface (WAI) has already
>>> helped this out considerably, I'm hoping there's something else we can do
>>> in this vein as well.
>>>
>>> Michael
>>>
>>> [1] http://hackage.haskell.org/package/http-api-data
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>>
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